Wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier-signals onto a single optical fiber by using different wavelengths of light. This technique enables bidirectional communications over one strand of fiber as generally separate wavelength sets are for different communication directions.
Access networks are presently experiencing rapid growth around the world. Both residential and business customers are demanding increasingly higher bandwidths from their Internet service providers who in turn are pressed to implement networks capable of delivering bandwidths in excess of 100 Mb/s per customer. For this application, passive-optical-networks (PON) are particularly well suited as they feature lowest capital-equipment expenditures relative to point-to-point and active optical networks. The book by C. F. Lam, Passive Optical Networks: Principles and Practice, Academic Press, 2007, and publication by C-H. Lee, W. V. Sorin, and B. Y. Kim, “Fiber to the Home Using a PON Infrastructure”, IEEE J. Lightwave Technol., vol. 24, no. 12, pp. 4568-4583, 2006 give good introduction into this technology. Wavelength division multiplexing in passive optical networks (WDM-PON) is one of the actively investigated as next-generation optical network architecture. WDM-PON potentially offers the lowest cost per unit of bandwidth to the user. However, the key difficulty in such a system has been the cost of the components, particularly arising from the need to transmit light at one wavelength for a specific channel and also receive information at any one of several other wavelengths at the user end in the so-called optical network unit (ONU). WDM optical and optoelectronic components traditionally exhibit high cost, among other issues, due to the requirement for precise wavelength adjustment in such systems. A dramatic cost reduction is achieved by eliminating wavelength-specific transceivers at the ONU in the colorless WDM-PON system, also referred to as a system with wavelength-agnostic transceivers in the ONU.
In a colorless optical network, the wavelengths emitted and received by the transceiver in the ONU are defined in the remote node or the central office rather than in the transceiver at the ONU as is well known in the art—see book by Lam cited above. The spectral filtering is done by an array-waveguide gratings (AWG) placed in the remote node and in the central office. AWG is a passive optical component that is ubiquitous in optical networking used for filtering, separating, combining, and routing signals of different wavelengths as is well known in the art. Its use and principle of operation is described in publicly available texts, such as, “WDM Technologies: Passive Optical Components” by A. K. Dutta, N. K. Dutta, and M. Fujiwara, published by Academic Press in 2003. The physical properties of the dielectrics used in building the AWG are temperature dependent and consequently this temperature sensitivity results in a shift of the filter wavelengths. It is well known in the art today that this temperature variation can be efficiently compensated resulting in so-called athermal array-waveguide gratings with very low temperature coefficient of filter wavelength drift. This technology is described in publicly available texts such as “Recent Progress on Athermal AWG Wavelength Multiplexer” by Shin Kamei published at the Optical Fiber Communications conference in San Diego, Calif. in 2009.
In PON architectures, information travels in two directions: downstream direction refers to information flow from the central office (OLT) to the end-user (ONU), while the opposite direction is referred to as upstream data flow. The primary cost concern in WDM-PON systems is related to the equipment placed at the user premises (ONU). Hence providing a reasonably priced colorless ONU to be placed at user premises is of high interest. The cost is furthermore reduced by reusing wavelengths: one wavelength or both upstream and downstream signals. Networks with wavelength-reuse are described in numerous publicly available texts such as B. W. Kim, “RSOA-Based Wavelength-Reuse Gigabit WDM-PON”, J. of the Opt. Soc. of Korea., vol. 12, no. 4, pp. 337-345, 2008, and S.-H. Cho, H. H. Lee, J.-H. Lee, J.-H. Lee, S.-I. Myong, S.-S. Lee, “A Loopback WDM-PON Technology Based on RSOA Using Wavelength Reuse Scheme and It's Commercialization Status in Korea”, 10th International Conference on Optical Internet (COIN2012), May 29-31, 2012, paper WF.1, Yokohama, Japan.
FIG. 1 illustrates an example of prior art in this field. An optical network using wavelength-reuse 100 comprises of a central office 101, trunk fiber 102, remote node 103, a distribution fiber 104, and at least one optical network unit 105 (also referred to as the client terminal). The central office 101 comprises of a multiplicity (only one is shown in FIG. 1) of optical sources 110 emitting a modulated optical signal that is passed through a circulator 112, coupled to one port of an array-waveguide grating 113 before it is emitted through the trunk fiber 102 over a distance to the remote node 103 where it is spectrally demultiplexed using an array waveguide grating 114 and one wavelength per source. This downstream optical signal is then routed to one of the exit ports of the array-waveguide grating 114 and then via distribution fiber 104 delivered to the ONU 105 where it is routed to an optical receiver 115 via an optical coupler 117. A portion of the downstream optical signal is coupled to an optical source 116 via the same optical coupler 117. This portion is used to define the emission wavelength of the upstream optical signal emitted from the optical source 116. The emitted upstream optical signal is coupled via optical coupler 117 to the distribution fiber 104 in the upstream direction, coupled to the AWG 114 at the remote node 103 and finally coupled to the central office 101 via the trunk fiber 102. At the central office 101, the upstream optical signal is passed though the AWG 113 and then directed towards the receiver 111 using the circulator 112. The optical signal emitted by the optical source 110 is modulated externally with downstream data, while the upstream optical signal emitted by the optical source 116 is detected and converted to electrical signal at the receiver 111. The element of network 100 that makes it a wavelength-reuse network is that the peak wavelength of the downstream optical signal is substantially equal to the peak wavelength of the upstream optical signal. In wavelength reuse architectures, the two wavelengths *upstream and downstream) are sufficiently close so that both of them fit into the same passband of same channel within the same diffraction order of the AWG.